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ECU GEOL 1500 - Deserts Processes and Landforms + Wind Generated Landforms - Notes - Fall 2014 - Copy

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DESERTS, PROCESSES AND LANDFORMS INARID AREAS, AND LANDFORMS FORMEDBY WIND EROSION AND DEPOSITIONDESERTSA Desert or Arid Climate Region is defined as a geographic area where annual rainfall is less than 10 inches (25 centimeters) or in which the potential evaporation rate exceeds the precipitation rate.Desert lands total about 25% of the land area of the world outside of polar regions. The Arid/Desert Climate Regions (<10 inches/year rainfall) and Semi-arid Climate Regions (10-20 inches/year rainfall or 25-50 cm/year) form a distinctive global pattern. Desert regions are not randomly distributed but are related to the Earth’s geography and atmospheric circulation. Based on differences in general geographic location and atmospheric circulationpatterns, Deserts are classified into 5 types. See notes and diagrams below:Type 1 – Subtropical Deserts (Hot) - The most extensive deserts are associated with 2 circum-global belts of dry, descending air located between latitudes 20o and 30o. (e.g., Sahara in northern Africa; Arabian and Lut Thar in Arabian Peninsula and Pakistan respectively; Great Australian Desert, also referred to as the “Great Sandy Simpson”.Type 2 – Continental Deserts (Hot) – This type of desert is found in continental interiors far from sources of moisture, where hot summers and cold winters prevail. (e.g., Gobi and Takla Makan in western China and Mongolia; Karakum in central Asia)Type 3 – Rainshadow Deserts (Hot)– This type of desert is found where a mountain range creates a barrier to the flow of moist air, forcing the air upward so that most of the moist air is precipitated out as airmasses cross a mountain range, thus leaving a zone of low precipitation on the lee side of major mountain ranges. (e.g., Patagonia Desert mostly in Argentina; Mojave, Sonoran, Chihuahuan, and Great Basin in SW and Western USA and northern Mexico)Type 4 – Coastal Deserts (Hot) – This type of desert occurs locally along the margins of continents where cold, upwelling seawater cools maritime airflowing onshore. (e.g., Atacama on west side of South America and Namib and Kalahari on west coast of Africa)Type 5 – Polar Deserts (Cold) – This type of desert occurs where precipitation is extremely low due to sinking cold, dry air. Polar Deserts differ from warm latitude deserts in that they are underlain by ice. (e.g.,dry valleys in Antarctica; Siberian Desert in northern Asia)Cover the answers and name the Deserts Yourself (might see on future exam)Desert Landforms Related to Weathering and Erosion by Mass WastingIn Arid and Semi-arid climate zones, Mechanical Weathering is dominant over Chemical Weathering. Mechanically weathered fragments of rock tend to break off along joints, leaving steep, rugged cliffs with 37-40o talus slopes or gently sloping pediments at their base.The most distinctive of these are called Plateaus, Mesas, Buttes, and Pinnacle/PedestalRocks (note descending size order). Also, due to less chemical weathering due to dryness of arid to semi-arid regions, regolith and soils are thinner, coarser, and less continuous, often with a calcareous K-Horizon. Desert slopes are steeper and more angular than Temperate-humid slopes because downslope creep processes are adjusted to coarser-grained regolith. Plateau – Extensive, flat-topped, steep-sided desert highlands, usually capped by some type of rock that is relatively resistant to weathering and erosion (much larger than a Mesa). Plateaus may occur in climate zones other than arid to semi-arid and these generally would not be as steep-sided.Mesa – A large, isolated, flat-topped, steep-sided desert table land, usually capped by some type of rock that is relatively resistant to weathering and erosion(larger than a Butte)Butte – A medium-sized, isolated, flat-topped, steep-sided desert hill usually capped by some type of rock that is relatively resistant to weathering and erosion (smaller than a Mesa)Pinnacle/Pedestal Rock – A small-sized, isolated, steep-sided desert hill/erosional remnant that remains as Buttes erode back by mechanical weathering and mass wasting processes. These are also capped by some type of rock that is relatively resistant to weathering and erosion and are considerably smaller than a ButteHigh Mesa, capped by Sandstone, near Abiquiu, NMButte, Capped by Sandstone near Abiquiu, NMAnd extensive Mesas in the backgroundPedestal or Pinnacle Rock, Pedestal Rock, Capped byCapped by Sandstone, Sandstone, in the Bisti De Na Near Abiquiu, NM Zen Wilderness, NMDesert Landforms Related to Deposition by StreamsAlluvial Fans – A fan-shaped body of Alluvium (= stream deposited sediments) that accumulates where a stream leaves a steep mountain valley. These usually have distributary channels but rarely any water flow in the distributaries due to high evaporation rates in arid areas and high rates of percolation/infiltration due to relatively high permeability. (See below)Compare Alluvium and Colluvium. Know the difference!Alluvial Fan at the Base of the Southern Rocky MountainsBajadas – A broad alluvial apron composed of coalescing alluvial fans. Formation of bajadas depends mostly on the spacing of canyons along a mountain front as well as time.Desert Landforms Related to Erosion by StreamsPediments – A gently sloping surface cut across bedrock or thinly veneered with alluvium that slopes away from the base of a mountain front in an arid to semi-arid region. (See Diagram above and Photo below)Pediment and InselbergPediment at the Base of the Sangre de Cristo Mountains, ColoradoInselbergs – Steep-sided mountains, ridges, or isolated hills that rise abruptly from adjoining plains or a rock island surrounded by an extensive flat area.(See diagram above and photo below)Inselberg – Ayers Rock, AUSTRALIAWind Erosion and Landforms in Arid to Semi-arid LandsDeflation (Erosional Process) – The picking up and removal of sand and dust by the wind.Desert Pavement (Desert Landform resulting from the process of Deflation) – A surface layer of coarse particles concentrated mainly by the process of Deflation.Desert Pavement in the Sahara DesertPans (Desert or Coastal Landform) – Wind eroded elliptical to circular areas of easily eroded sediment (sand and silt) that are form in dune fields in arid and coastal environments. These depressions are usually elongate in the direction of the prevailing wind.Playa/Playa Lakes (Desert Landform)– A dry lake bed characterized


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